In the field of a blood transfusion, in addition to the so-called whole blood transfusion that a whole blood preparation prepared by adding an anticoagulant to blood gathered from donors is transfused, a so-called blood component transfusion has heretofore been generally carried out by which the blood component which the recipient requires separated from the whole blood preparation is transfused into the recipient. The blood component transfusion includes erythrocyte transfusion, platelet transfusion, plasma transfusion and the like depending upon the kind of blood component required by a recipient, and the blood component preparations used in these transfusions include erythrocyte preparation, platelet preparation, plasma preparation and the like. Recently, a so-called leukocyte-removed blood transfusion has been spread by which a blood preparation is transfused after the leukocyte mixed in the blood component preparation has been removed. This is because it has been clarified that relatively light side effects such as headache, nausea, chill, non-hemolytic febrile transfusion reaction and the like which accompany the blood transfusion; grave side effects such as alloantigen sensitization, virus infection, post-transfusion GVHD and the like which seriously affect the recipient are caused mainly by the leukocyte mixed in a blood preparation used in the blood transfusion.
In order to prevent the relatively light side effects including headache, nausea, chill, fever and the like, it is said that the removal of the leukocyte in a blood preparation until the proportion of the residual leukocyte becomes 10.sup.-1 to 10.sup.-2 or less is sufficient. Also, in order to prevent the alloantigen sensitization and virus infection which are grave side effects, it is said that the removal of the leukocyte until the proportion of the residual leukocyte becomes 10.sup.-4 to 10.sup.-6 or less is sufficient.
The method of removing leukocyte from a blood preparation is roughly classified into two methods, one of which is a centrifuging method by which leukocyte is separated and removed by utilizing the specific gravity difference of blood components by using a centrifugal separator and a filtering method by which leukocyte is removed using a filter material consisting of a porous element such as a fibrous material, a porous material having interconnected cells or the like. The filtering method has such advantages as excellent leukocyte-removing performance, simple operation, low cost, etc., so that the filtering method has been spread. Moreover, as the filtering method, a method which comprises removing leukocyte by sticking or adsorbing using a nonwoven fabric as a filter material is now the most spreading because this method is particularly excellent in leukocyte-removing performance.
The mechanism of removing leukocyte by a filter apparatus using the above-mentioned fibrous material or porous material is mainly attributed to the fact that the leukocyte which has contacted with the surfaces of the filter material sticks to or adheres to the surfaces of the filter material. For example, EP-A-0155003 discloses a technique by which a nonwoven fabric is used as the filter material. Furthermore, WO93/01880 discloses a leukocyte-removing filter material produced by dispersing in a dispersion medium a mass of a great number of small fiber pieces having a fiber diameter of not more than 0.01 .mu.m and a length of about 1 to 50 .mu.m, together with spinable and weavable short fibers having a fineness of about 0.05 to 0.75 d and an average length of 3 to 15 mm, and removing the dispersion medium from the resulting dispersion.
The existing leukocyte-removing filter has such a leukocyte-removing performance as to decrease the number of the residual leukocytes to not more than 1.times.10.sup.5. Under such circumstances, two demands for the leukocyte-removing filter have been brought up in the market.
The first demand is to enhance the recovery of the useful component and improve the handling by rendering the operation of recovering the useful component remaining in the filter and the circuit unnecessary by the presence of a physiological saline solution and air. In particular, blood which is the starting material for the blood preparation is in many cases a precious one supplied by well-intentioned blood donation, and the blood which has remained in the filter and has become impossible to recover is scrapped as it is, together with the filter and goes to waste. Therefore, it is very significant to increase the recovery of the useful component as compared with the existing leukocyte-removing filter. However, in the case of the leukocyte-removing filter using the conventional technique, it is difficult to very greatly increase the recovery of the useful component.
The second demand is to achieve a higher leukocyte-removing rate than the existing leukocyte-removing filter and completely prevent a grave side effect from being caused by the leukocyte transfused into a patient. However, with the leukocyte-removing filter using the conventional technique, it is difficult to achieve so high a leukocyte-removing rate that such a side effect can be completely prevented.